The present invention relates to a sealing jaw for manufacturing a sealing seam in a heat-sealable packaging material. The invention furthermore relates to an induction sealing device with a sealing jaw and to a packaging machine with induction sealing device.
Many flowable food products such as, for example, fruit juice, pasteurised or UHT milk, wine, tomato sauce and so forth are purchased in packages that are manufactured from sterilised packaging material.
Cuboidal milk packages are known that are formed by folding and sealing laminated packaging material with a multi-layered structure that has a fibrous layer, for example, paper, that is covered on both sides with layers of heat-sealable plastics material, for example, polyethylene. In many applications, in particular in the case of sterile packages of products with very long shelf-life such as, for example, orange juice or tomato sauce, a packaging material is generally used that has a barrier layer, for example, an aluminium layer, that is applied to a layer of heat-sealable plastics material, and is itself in turn covered with another layer of heat-sealable plastics material.
Such packages are typically manufactured and filled on fully automated tubular bag packaging machines. The packaging material web is sterilised in the packaging machine, for example, by the application of a chemical sterilising agent such as, for example, a hydrogen peroxide solution. The chemical sterilising agent is removed after sterilising. This can be done, for example, by vaporising the agent by heating the surface of the web. The sterilised packaging material is then kept in a closed, sterile environment and is folded and sealed in the longitudinal direction in order to form a continuous tubular bag.
This tubular bag is moved continuously in a first, usually vertical direction, is filled with the sterilised or sterile processed foodstuffs, and is then gripped by two pairs of jaws that heat and press the packaging material so that surface melting of adjacent thermoplastic layers is obtained in order to produce a fluid-tight weld. This right-angle weld is frequently also referred to as a right-angle sealing seam. For welding or sealing in this manner, so-called heating or sealing jaws are usually used that receive the packaging material to be sealed in between them and can press together at the same time as being heated, such that they melt the plastics layers that are pressed together between the sealing jaws and are lying beside one another, and form a sealed and thus a mechanically strong weld point. The pair of jaws can be formed either by two sealing jaws that both have a respective heating device, or just a single sealing jaw can be used that is pressed against a counter-jaw that serves as a thrust bearing.
Sealing jaws are often used that have thin metal strips that are arranged on an insulating material, usually a ceramic material. In order to heat the metal strips, a short current pulse is conducted through the strips, which are thereby very rapidly heated to high temperatures. The heat generated in the metallic strips is transferred to the packaging material because of the direct contact connection, so because of the pressure that is exerted by the pair of jaws, layers of packaging that touch are fused together.
The sealing jaws usually have two approximately parallel heating strips that produce two corresponding right-angle sealing seams, so with the aid of a cutting or separating device, the packaging material can be separated between the two right-angle sealing seams in order to obtain individual, sealed packages.
As described hereinabove, the section of the packaging tube that is clamped between the pair of jaws is melted by the heating device attached to at least one of the jaws, so the two layers of heat-sealable plastics material that are held firmly between the jaws melt locally.
Where a packaging material is used with an electrically conductive layer such as, for example, aluminium, the packaging material can also be sealed by using a so-called induction heating process wherein when the tube is pressed together by the jaws, an eddy current is induced in the aluminium layer in order to locally heat the aluminium layer and thereby to locally melt the heat-sealable layers that are adjacent to the aluminium layer.
In these instances the heating device for the sealing jaw is formed by an inductor to which a high frequency voltage can be applied. The inductor is usually composed of one or more induction bars that are manufactured from electrically conductive material and extend parallel to the right-angle sealing seam to be produced. After the tubular packaging material has been clamped between the jaws fitted with inductors, an eddy current can be induced by means of the inductor in the tubular packaging material, and the material is heated to the necessary temperature for sealing. In this configuration, the other jaw usually does not have an inductor, but simply has pressure plates, usually of an elastomeric material, that cooperate with the induction bars.
Such a sealing jaw with an induction device is known, for example, from WO 00/20279.
When filling with flowable foodstuffs that contain small solid particles such as, for example, pips or fibres (of fruit pulp) it may happen that the solid particles are found between the jaws during melting of the right-angle sealing seam. This can lead to the right-angle sealing seam being less durable, or in the worst case even leaking from the outset. Such a faulty right-angle sealing seam is usually not noticed immediately during the packaging process, however. The leakage occurring is usually so slight that initially the packaged product does not leak from the package. Nevertheless, bacteria present in the environment go through the faulty right-angle sealing seam into the product. This can lead to fermentation and decay processes in food products, whereby the pressure within the package significantly increases and sooner or later the right-angle sealing seam bursts. Even when this affects only a proportion of less than 0.1% of packages using the conventional sealing procedure, a defective right-angle sealing seam can have considerable disadvantageous consequences, in particular, when as is often the case the completed packages are placed on pallets in large numbers. If a right-angle sealing seam is leaking, the contents of the faulty package leak over all the packages lying below it, so often even the undamaged packaged are virtually unsaleable because of their soiled outsides.